Two-phase flow instabilities are challenging issues hindering the practical implementation of flow boiling in micro domains. In particular, parallel channels instability, upstream compressible volume instability, and the critical heat flux (CHF) condition are common at the micro scale. The former two are characterized by low frequency and large temperature and pressure drop fluctuations. Flow instabilities can induce local dry-out between heated walls and attached vapor bubbles, and hence, lead to premature CHF conditions at low exit mass quality. Recent studies demonstrated that configurations of microchannels can suppress boiling instabilities and enhance several key flow boiling parameters including onset of nucleate boiling (ONB), heat transfer coefficient (HTC) and CHF. An example of reported configurations is inlet restrictors (IR) or valves/orifices to overcome reverse flows and mitigate boiling instabilities. Several studies have shown that other configurations, such as impingement jets, can effectively suppress flow reversal and at the same time enhance HTCs and CHFs.
Although the aforementioned techniques successfully enhanced nucleate boiling and suppressed the intrinsic instabilities during flow boiling in micro scale systems, the additional pressure drop (Δp) and power requirements introduced by those configurations are a major hindrance. For example, inlet restrictors or orifices can effectively suppress flow instabilities and enhance CHF, but, they increase the pressure drop by up to five-fold and cannot enhance flow boiling heat transfer rate. High flow resistance can cause critical issues, such as high pumping power and unpredictable coolant leakage. Similarly, comparing to microchannels with plain walls, the requirements in additional pumping power, arrangement of jets, and the availability of proper pumps have hindered application of impingement jets. In summary, considering reliability issues and cost would impede practical applications of current practices.
Two-phase flows in microchannels are dominated by viscous and capillary flows, and thus, mixing can considerably enhance flow boiling heat transfer in microchannels. However, it is challenging to passively generate strong mixing in microchannels because of the aforementioned reasons. The reported two-phase oscillation (TPO) frequency ranges from 0.06 Hz to 2 Hz in the laminar flow regime without assistance of active controllers and actuations. To date, high frequency TPOs can only be achieved through local microsecond heating pulses with significant sacrifice of volumetric flow rates. The non-sustainable water hummer pulses driven by bubble growth and collapse process have been observed and modeled in a single microchannel.